1. Field Of The Invention
This invention relates to amorphous metal strips having a large thickness with good magnetic properties and a method for producing the same, and more particularly to amorphous metal strips having a large thickness produced by a melt spin process wherein a stream of molten metal is quenched and solidified on the peripheral surface of a rotating annular chill roll.
2. Description Of The Prior Art
Iron based alloys that are rapidly solidified to thin strip with an amorphous microstructure are known to have interesting soft magnetic properties, making them attractive as highly efficient cores for electric transformers. The casting of strip having an amorphous structure requires cooling rates of 10.sup.6 .degree. C./sec to avoid crystallization and deterioration in the desired magnetic properties, this limits the thickness of the strip. The composition of iron based alloys and the required casting conditions are described in detail in U.S. Pat. Nos. 3,856,513, 3,862,658 and 4,332,848. U.S. Pat. No. 5,496,418 discloses rapid solidification casting of amorphous metal strips, the thickness of which is limited to 25 .mu.m. Amorphous metal strips are produced on a commercial scale by AlliedSignal Inc. and marketed under the METGLAS.RTM. trademark. The strips are produced by the Planar Flow Casting process described in U.S. Pat. No. 4,142,571 and have a thickness of approximately 25 micrometers (.mu.m). At this thickness the alloys find uses predominantly in low power wound core distribution transformers. Strip ductility, or the ability to handle strip in the transformer core making process, is the primary factor limiting the thickness. Thicker strip is required for higher power stacked core transformers.
The thinness of the amorphous strip makes handling difficult in comparison to the thicker FeSi sheet that is currently used for transformer laminations in stacked core transformers. Specifically, when the thin amorphous metal strip is stacked into a transformer the extra laminations required to fill the same space increase production costs. In addition, the increased number of air gaps between laminations decreases the packing density, commonly referred to as lamination factor or space factor, reducing the transformers' efficiency. Accordingly, there is a need for thicker amorphous metal strip with low magnetization losses and exciting power. The thicker strip must be ductile enough to be handled during manufacture of transformer cores.
Considerable research has focused on the Planar Flow Casting process to produce thicker strip. In this process, the alloy melt is delivered through a slotted nozzle into a stable puddle maintained between the slot lips and a moving substrate. This stable puddle is the unique feature of the process. All process parameters for a given casting apparatus are adjusted to preserve stability. These parameters are: nozzle slot width, nozzle-to-substrate distance ("casting gap"), melt ejection ("casting") pressure, and substrate speed, all of which, in concert, control the puddle length. This length, limits the time available for the solidification of the glassy strip and, therefore, governs the strip thickness. While it may seem apparent that changing one of these parameters to increase melt flow rate would increase the strip thickness, the dynamics of the process are such that the puddle integrity could be seriously compromised. Strip with poor surface quality is the result; at the extreme, the puddle "blows out".
Surface quality impacts the practical application of amorphous metal strips in multiple layer configurations such as transformer cores by its effect on packing density. The rougher the strip, that is, the more the local strip thickness varies along its width and length, the greater the volume that is filled with a given number of layered strips. The cost of devices such as transformers which utilize cores made from multiple layers of amorphous alloy strip is strongly related to the physical size of the cores.
The packing density of amorphous metal strip, also referred to as lamination factor, stack factor or space factor, is described by the quantity equal to the weight density of a stack of amorphous metal strips divided by the weight density of the strip material comprising the stack. A high lamination factor, preferably greater than about 0.80, is desirable for use of amorphous metal alloys in transformers as it allows a physically smaller core to be constructed for a given performance level.
It is well known that the mechanical properties of an amorphous metal strip depend on the sheet thickness. As strip thickness increases, the heat that must be extracted in order to solidify it increases, thereby decreasing the cooling rate. This decrease in cooling rate is accompanied by a decrease in strip ductility and handleability. A common measure of strip ductility is fracture strain. Fracture strain, .epsilon..sub.f, is usually represented by the expression .epsilon..sub.f =t/(2r-t), wherein t is the strip thickness and r is the bending radius at which fracture occurs. In general, a high fracture strain is desirable; for practical use of amorphous metal strip the fracture strain should be greater than 0.01.
Magnetic properties of amorphous alloy strips are also known to depend on thickness. Amorphous metal strip typically requires an annealing treatment to optimize magnetic properties such as core loss and exciting power for transformer applications. The specific annealing conditions may vary depending on factors such as specific alloy composition, strip configuration, and transformer design considerations, but typically involves heating the strip to between 350.degree. C. and 400.degree. C. for between 60 minutes and 180 minutes. In general, core loss is not strongly affected by an increase in thickness as long as it remains substantially amorphous. As thickness increases, however, the cooling rate decreases until a critical value is reached at which substantial crystallinity is formed. At that point, losses begin to increase rapidly with thickness.
Earlier attempts to produce thick strip involved using a belt as a quench substrate [Electric Power Research Institute Report, EPRI TR-101978, April 1993]. Belt casting trials to produce thick strip failed because of a lack of ductility in the as-cast strip. The thick, amorphous strips were otherwise magnetically acceptable. In this study, a moving belt was used as the substrate, which was cooled by a water spray. A major reason for the employment of a belt as the quench substrate was that a belt approximates a "wheel" of infinite diameter, so that low substrate return temperatures could be maintained even when casting a thick strip. However, deficiencies in the heat extraction ability of the apparatus and belt distortion were the primary reasons for failure to produce thick ductile strip.
Other efforts have been made to develop techniques that increase the thickness of the strip, while maintaining an amorphous structure. One such technique is described in U.S. Pat. No. 4,782,994, in which strips are bonded together. Although bonding of thin strips maintains reasonable magnetic properties, such bonded strips are inherently brittle.
U.S. Pat. No. 4,865,664 and U.S. Pat. No. 5,301,742 disclose processes in which thicker (to 100 .mu.m) amorphous metal strip is cast via the use of a nozzle having a plurality of slotted openings spaced slightly apart from each other. The methods disclosed therein involve a cumbersome process of drawing out a molten metal on the moving chill substrate through a first molten metal puddle portion to make a first strip; drawing out a second molten metal over the first strip in a not completely solidified state through a second molten metal puddle portion so as to make a second strip; and drawing out subsequent molten metals through further portions so as to make subsequent strips until the required sheet thickness is obtained. Use of this method is said to produce an amorphous metal strip greater than 50 .mu.m thick having a room temperature fracture strain greater than 0.01, a lamination factor greater than 0.85 and good magnetic properties. U.S. Pat. No. 4,865,644 and U.S. Pat. No. 5,301,742 disclose further that amorphous metal strips having a large thickness with similarly good properties can not be produced using a single slotted nozzle.
It would be advantageous if amorphous metal strips having large thickness and good structural and magnetic properties could be produced on a single roll casting apparatus using a single slotted nozzle. Such a product and the process for producing it would be highly desirable, especially for the production of wide strip, owing to the ease of manufacture and robustness relative to processes wherein the nozzle has multiple slots.
There remains a need in the electric transformer art for thicker amorphous strip having physical properties, including ductility and lamination factor, adequate for the manufacturing of transformer cores and having magnetic properties after annealing similar to those of 25 .mu.m thick amorphous strip presently in use.